1. Field of the Invention
The invention relates to an optical coupling semiconductor switching circuit which is operated by an optical signal.
2. Description of the Related Art
An optical coupling semiconductor switching circuit in the related art, which is operated by optical signals, includes a light detector, an impedance circuit, and a semiconductor switching device such as a FET transistor that is connected to electrical circuits. When the light detector receives light from a light emitting device, the light detector changes a light signal to an electric signal to generate voltage. The voltage, which is output from the light detector, is input to the impedance circuit. The semiconductor switching device is operated when the semiconductor switching device receives the voltage as a gate voltage from the light detector via the impedance circuit.
Since the impedance circuit is in a high impedance state when the voltage is applied to an input terminal of the impedance circuit, the voltage at the input terminal can be output effectively from first and second output terminals of the impedance circuit with little loss. Therefore, the semiconductor switching device whose gate is connected to the first output terminal is operated without fail. Further, while the light detector does not receive light so that the voltage is not applied to the input terminal of the impedance circuit, the impedance circuit is in low-impedance state. Thus, the connection of the impedance circuit is short-circuited between its output terminals. In this situation, as the charge at the gate of the semiconductor switching device rapidly discharges, the gate voltage falls rapidly. Therefore, speedy operation of the semiconductor switching device can be performed.
In the related art, it is required that a plurality of electrical circuits are operated by a single light signal. In this case, the same number of optical coupling semiconductor switching circuits as electrical circuits is formed. Each optical coupling semiconductor switching circuit has a light detector, an impedance circuit and a switching device, and is connected to respective electrical circuit. Thus, the overall circuit is both large and complicated.